1. Field of the Invention
This invention relates to an optical fibre cable of use inter alia for submarine transmission systems. It comprises a cylindrical core member having a periphery with continuous helical grooves, each receiving at least one optical fibre and a composite tubular sheath having a first inner jacket closing the grooves.
2. Description of the Prior Art
Optical fibre submarine cables of this kind are disclosed e.g. by French Patent Application No. 2,401,434, United Kingdom Patent Applications Nos. 2,001,777 A; 2,107,967 A and 2,021,282 A and by an article by F. F. Gleason, R. C. Mondello, B. W. Fellows and D. A. Hadfield entitled: "Design and manufacture of an experimental lightguide cable for undersea transmission system" published in the Proceedings of the 27th International Wire and Cable Symposium, Cherry Hill, New-York, U.S.A., 14-16 November 1978, pages 385 to 389.
As a rule a submarine optical fibre cable core member takes the form of a rod which can be metal, e.g. copper, or extruded plastics. The periphery of the core member can be formed with helical grooves each of which contains one or more optical fibres; alternatively the core member can be embodied by V-shaped folded splicing tapes of polyester or aluminium which are welded together to make up a circular transverse cross-section and each of which contains one or more optical fibres; or else the central member can be embodied by a steel wire around which optical fibres are completely encapsulated in an elastomeric buffer which is sheathed with a fine layer of nylon by means of a tubing extrusion operation. A composite sheath for the core comprises, in addition to a plastic outer insulating jacket around a first metal jacket, an inner protective strength tube. This strength tube is lodged inside the last-mentioned two jackets and is embodied by a plurality of layers of steel wires which are stranded around the central core member.
Since the strength tube extends around the central core member and is therefore of greater diameter than the latter, the resulting cable is relatively bulky and heavy per unit length, with a consequent limitation in the number of optical fibres, i.e. in cable capacity.
All of these known kinds of cable have the main disadvantage of causing the optical fibres to experience substantial and uncontrolled stresses, for the high underwater pressures, of the order of some 6 to 8 daN/mm.sup.2, make it difficult to give credence to the idea of a "strength" tube formed by wires or cables serving for remote feeding of the transmission system repeaters. The strength tube is also required to have very high tensible strength and is correspondingly embodied by strands of high-strength steel--a very expensive form of tube construction. The non-uniformity of such an assembly and the possible deformations lead to very severe stresses being imposed on all the optical fibres during manufacture of the tube and during the laying and operation of the cable. The carrying portion of the cable in the form of the stranded wires around the central member is bound to become distorted at least during cable laying. Measurements made on optical fibre transmission cables of this kind show that the fibres are very sensitive to such effects, which include greatly increased attenuation and, in the fairly long term, a risk of fibres rupturing.
Also, since the strength tube extends around the optical fibres, the diameter of the central core member, and therefore the number of optical fibres, has to be limited. This consideration also impedes connection of the cable to the repeaters and repair of the cable in the event of the optical fibre breaking. Also, the internal arrangement even of the elements included in the cable does not make for ready manufacture of long cable runs, since the operations of forcing on the strength tube and of multistage sheathing or extrusion encapsulating around the central core member supporting the optical fibres may cause breakings thereof. Welding and storage operations for the butting together of unit cables are also difficult matters.
With a view mainly to obviating the disadvantages of the protective strength tube around the central core containing the optical fibres, U.S. Pat. No. 4,199,224 uses as a cable-strengthening element a central core member having a center embodied by a plurality of layers of steel wires similarly to the core of conventional coaxial submarine cables. The central strand of steel wires is surrounded by a radial system which comprises chambers and which is an extruded plastic section member having a cross-section the shape of a spoked wheel. Each chamber extends helically around the central steel strand, is open radially towards the exterior and receives at least one optical fibre.
The composite tubular cable sheath in accordance with the U.S. Pat. No. 4,199,224 has, as inner jacket extending around and closing the chambers or compartment, a composite crepe paper and/or plastic. The material used for this inner jacket, the presence of the chamber system and the structure thereof make it impossible for a cable of this kind to be used as a submarine cable, for the following reasons.
First, the high submarine pressures acting on such a cable would crush the plastics chamber system. The height and width of a transverse cross-section of chambers of this cable are typically 6 mm and 5 mm and the chambers are spaced apart from one another by radial ribs which are 6 mm high and only 2 mm thick. Second, the damage would be aggravated because the inner jacket of the composite sheath is of the order of 1 mm thick and is made of plastic. The compression strength of this jacket is at least of the order of 11 daN/mm.sup.2, a result achieved by using a high compression strength material, such as loaded polystrene. Each rib therefore must withstand an underwater pressure of from 6 to 8 daN/mm.sup.2 over approximately 7 mm of a cylindrical sector of the inner jacket, i.e. the compression strength of each 2 mm thick rib must be approximately (6.times.7)/2=24 daN/mm.sup.2 to (8.times.7)/2=28 daN/mm.sup.2, very much more than 11 daN/mm.sup.2.
Also, the chambers of the radial system are completely full of a non-freezing substance which surrounds the optical fibres. The sheath inner jacket is therefore in direct contact with such substance and compression thereof therefore helps to crush and damage the optical fibres.
In a variant of U.S. Pat. No. 4,199,224, a layer of electrical conductors for remote feeding of the transmission system repeaters is provided between the two tubular elements forming the composite inner jacket of the cable sheath. This single layer of conductors is not a strength tube for underwater cables in the sense hereinbefore defined and would also help to crush the radial chamber system if the cable were used underwater.